The invention relates to a film scanner comprising a first scanning device for scanning frames of a cinematographic film by means of photoelectric transducers, and a second scanning device for scanning the sprocket holes. The invention also relates to scanning devices as such and to a method of scanning the sprocket holes in a cinematographic film.
To convert cinematographic film material into electronic signals, a film in a film scanner is passed along or through an opto-electronic scanning device. This poses the long-standing problem of maintaining the frame position of successively scanned frames constant. The partly periodical, partly statistic fluctuations of the frame position, which are referred to as picture steadiness errors or film weave, may have different causes. These errors may be, for example positioning errors in both the pick-up camera and in the negative/positive copying machine. However, also frame position errors and tracking errors of the film scanner may lead to further steadiness errors.
To reduce picture steadiness errors, there are different formulations for solutions as described, for example, in DE 38 19 496 C2 in which a sprocket hole which is assignable to a film frame is scanned by means of a cathode ray generated by a cathode ray tube and serves as a reference point for each scanned frame. Dependent on the position of a sprocket hole determined in this manner, a correction signal for correcting the horizontal lateral irregularity is generated. Here, the problem is encountered that in given film material the quantity of light passing through the film is approximately the same as the quantity of light transmitted through the sprocket hole. This problem occurs, for example in negative film material which has an orange basic color. To solve this problem, it is proposed to use green-sensitive photocells for scanning the sprocket hole in this case, because these cells, as compared with the red and blue sensitive photocells, supply a signal with the smallest electronic interference.
A problem in film scanners with optical scanning of the sprocket hole is that the first scanning device for scanning the film frames may be disturbed by light scattered by the second scanning device for scanning the sprocket holes. In fact, the operations of scanning the sprocket holes and scanning the frame are preferably performed simultaneously at the same location, which presupposes a narrow spatial proximity of the areas where light occurs and passes through for detecting the sprocket holes and for scanning the frames. It is ensured only by the narrow spatial and temporal proximity of the two scanning operations that the offset errors determined during the operation of scanning the sprocket holes also correspond to the offset errors of the scanned frames. However, such a spatial proximity led to problems with scattered light in known scanning operations.